The program of research outlined in this proposal extends our analysis of associative learning in the marine mollusc, Aplysia, an animal which is particularly advantageous for examining different forms of learning at the cellular level. Previously we have found that the gill- and siphon-withdrawal reflexes -- two simple and related behaviors whose neural circuitry is will understood -- are capable of two associative forms of learning, classical and operant conditioning, as well as two nonassociate forms of learning, habituation and sensitization. We have also found that a mechanism of classical conditioning of the withdrawal reflex appears to be an elaboration of a mechanism of sensitization of that reflex on the cellular and molecular levels. We now plan, in the proposed research, to use this system to extend our analysis of associative learning in four new directions: 1) a further analysis of molecular mechanisms of classical conditioning in the isolated nervous system, 2) an analysis of cellular mechanisms of classical conditioning in a semi-intact preparation, 3) a cellular analysis of higher-order features of classical conditioning, and 4) an analysis of cellular mechanisms of operant conditioning and comparison with classical conditioning.